The present invention relates to probe positioning and drift compensation for scanning tunneling microscopes, atomic force microscopes, and the other scanning probe microscopes; and, more specifically, in a scanning system wherein a probe carried by positioning apparatus is positioned in at least two perpendicular dimensions by the application of a positioning signal from a positioning generator to the positioning apparatus for each of the two dimensions, to apparatus for providing supplemental movement to the probe comprising, a supplemental signal generator providing supplement signals to the positioning apparatus which supplemental signals define supplemental motion and which supplemental signals may be simultaneously applied to the positioning apparatus in conjunction with positioning signals.
Scanning Probe Microscopes are instruments that provide high resolution information about the properties of surfaces. One common use of these devices is imaging, and some types of SPM have the capability of imaging individual atoms. Along with images, SPMs can be used to measure a variety of surface properties, with detail over the range from a few angstroms to hundreds of microns. For many applications, SPMs can provide lateral and vertical resolution that is not obtainable from any other type of device.
The first type of SPM developed was the scanning tunneling microscope (STM). The STM places a sharp, conducting tip near a surface. The surface is biased at a potential relative to the tip. When the tip is brought near the surface, a current will flow in the tip due to the tunneling effect. Tunneling will occur between the atom closest to the surface in the tip and the atoms on the surface. This current is a function of the distance between the tip and the surface, and typically the tip has to be within 20 angstroms of the surface for measurable current to be present. An STM has a mechanism to scan the tip over the surface, typically in a raster pattern. While the tip is scanned over the surface, the tip is kept at a constant distance above surface features by means of a feedback loop employing the tunneling current and a vertical position controlling mechanism. The feedback loop adjusts the vertical position of the tip to keep the tunneling current, and thus the distance, constant. The vertical position of the tip is determined from the control signals applied to the vertical position controlling mechanism. The vertical position, as a function of horizontal scan position produces a topographic map of the surface. STMs can easily image individual atoms, and can also be used for highly accurate surface measurements of larger scale, up to a few hundred microns. STMs also may be used for data other than topographic images. One alternative operation of an STM is to hold the tip stationary while varying the bias voltage applied to the sample and monitoring the tunneling current, thus measuring local current/voltage characteristics of the surface. STMs require a conducting sample surface for operation. Insulating surfaces may be coated with a thin conducting material such as gold, or in some cases, insulating materials a few molecules thick lying on a a conducting surface may be imaged.
Another SPM, the atomic force microscope (AFM), similarly scans a tip across a surface. The tip in this case is mounted on the free end of a lever or cantilever which is fixed at the other end. The tip is brought to a surface such that the force interaction of the tip with the surface causes the cantilever to deflect. A feedback loop employing the cantilever deflection information and the tip or sample's vertical position can be used to adjust the vertical position of the tip as it is scanned. The feedback loop keeps the deflection, and thus the force, constant. The tip vertical position versus horizontal scan provides the topographic surface map. In this mode, the forces on the surface can be made very small, so small as not to deform biological molecules. AFMs can also be operated in a mode where the repulsive force deflects the cantilever as it scans the surface. The deflection of the tip as it is scanned provides topographic information about the surface. AFMs may also be operated in a non-contact mode where the cantilever is vibrated and the Van der Waals interaction between the tip and surface can affect the vibration frequency or amplitude. AFMs have a means to detect the small movements of the cantilever. Several means for cantilever motion detection have been used with the most common method employing reflected light from the cantilever. The deflection of a light beam due to the cantilever motion may be detected, or the movement of the cantilever can be used to generate interference effects which can be used to derive the motion. Like an STM, AFMs can image individual atoms; but, unlike STMs, AFMs can be used for non-conducting surfaces. AFMs may also be used for measurements such as surface stiffness.
Other SPMs may use different probing mechanisms to measure properties of surfaces. Probing devices have been developed for such properties as electric field, magnetic field, photon excitation, capacitance, and ionic conductance. Whatever the probing mechanism, most SPMs have common characteristics, typically operating on an interaction between probe and surface that is confined to a very small lateral area and is extremely sensitive to vertical position. Most SPMs possess the ability to position a probe very accurately in three dimensions and use high performance feedback systems to control the motion of the probe relative to the surface.
The positioning and scanning of the probe is usually accomplished with piezoelectric devices. These devices expand or contract when a voltage is applied to them and typically have sensitivities of a few angstroms to hundreds of angstroms per volt. Scanning is implemented in a variety of ways. Some SPMs hold the probe fixed and attach the sample to the scanning mechanism while others scan the probe. Piezoelectric tubes have been found to be the best scanning mechanism for most applications. These tubes are capable of generating three dimensional scans. They are mechanically very stiff, have good frequency response for fast scans, and are relatively inexpensive to manufacture and assemble. Such scanners are used in commercial STMs and AFMs sold by the assignee of this Application, Digital Instruments Inc., under the trademark NanoScope. These scanners are made in various lengths, the longer ones having larger scan ranges.
FIG. 1 is a simplified block diagram of a typical scanning probe microscope 10. The probe tip 12 is positioned by a piezoelectric scanner 14 over a stationary sample 16; or, in some cases the sample 16 is attached to the scanner 14 and the tip 12 is stationary. The controller 18 acquires data from the sensing device 20 and through feedback controls the height of the tip 12 by applying control voltages to the scanner 14 through the z position driver 22. The sensing device 20 senses tunneling current in the case of an STM, tip deflection in the case of an AFM, or other parameters for other scanning probe microscopes. The x and y positions are controlled by applying voltages to the scanner through the x and y drivers 24 and 26. Typically for most applications, a raster scan is generated by producing a linear motion in the x and y scan directions. The scan area can be offset by starting the raster from a selected position within the scanner range. The probe tip 12 in this arrangement can be positioned anywhere in x and y within the range of the scanner.
In existing scanning probe microscopes, drift of the probe tip across the sample is a significant effect. The drift distorts the image and also makes it difficult to continue imaging the same feature over time. Typically, drift in the x-y plane is several angstroms per minute after the set-up has stabilized. Drift can be much greater when a sample is first contacted, sometimes requiring several hours of stabilization before accurate scanning can occur. Drift is due to thermal expansion of the piezoelectric scanner as well as the sample itself and its holder. Additional drift contributions come from "creep" and hysteresis of the piezoelectric material. Some drifts, such as the drift due to thermal effects, are long-term and are typically constant over the scanning of single images. Other drifts, such as the drift due to piezoelectric creep, have time dependences on smaller scales. Existing designs attempt to minimize the drift by mechanical means, such as matching thermal coefficients for probe materials or using materials that are very stable. For instance, the most widely used STM uses Invar in its construction to minimize thermal drifts. The reliance on mechanical means to reduce drift, however, may impose restrictions on the configuration of the system which severely decrease its flexibility. Most existing general purpose designs typically achieve long-term drift velocities as low as a few angstroms per minute. It can be seen that for a typical atomic feature whose size is on the order of 10 angstroms, after a few minutes the feature will have shifted significantly in the image and in a short time will have moved out of the scan area. Short-term drifts, particularly at the beginning of scanning or after a scan area has been offset, can be of much greater magnitude and can cause image distortions on the order of the scan size. One could attempt to correct the drift effect after the data is collected, which can improve the visual quality of an image. Off-line correction, however, cannot address the problem of drifting off of a desired feature or distortions that cause features to be improperly placed either in or out of the scan area.
Therefore, drift in scanning probe microscopes restricts their ability to dwell on atomic dimension features, an ability that would be extremely useful for monitoring local processes or acquiring repeated images of unique structures. Drift also can cause inordinately long stabilization times before undistorted images can be acquired for larger images.
What is needed therefore, is an SPM which possesses the ability to generate a controlled motion of the scan area that is independent of the scanning motions and can be used to cancel the drift motion. The ability to add a controlled motion could be useful for other reasons as well. In some applications, e.g. when imaging large areas, it would be useful to move the sample at a constant velocity and also scan it while it is moving without distortion. Successive frames would smoothly and continuously image different areas of the sample. It is therefore desirable, for many reasons, to provide motion that is independent of the scanning motions for scanning probe microscopes.
Wherefore, it is an object of this invention to provide a capability for STMs, and the like, that is not dependent on the physical design of the scanner or sample holder for adding a controllable supplemental motion to the probe tip.
It is another object of this invention to provide a drift compensation capability for STMs, and the like, wherein compensation motion is independent of the raster scan or other positioning of the tip and can be a constant motion or may vary with time.
It is yet another object of this invention to provide a compensation capability for STMs, and the like, wherein the compensation motion can be used to cause the scan frame to follow the sample, whether relative motion between the sample and scan frame is unwanted drift or some desired motion.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.